Vacuum casting is the process of using the ventilation mould. Molten metal relies on air pressure to flow into the mold and then clears the air to form a vacuum. This casting method is mainly used for small parts or jewelry with exquisite detail.
Vacuum mould is a kind of physical modeling method, which combines vacuum technology with sand mould casting, rely on plastic film will be sand mold cavity surface and back sealing up, with the help of vacuum pump suction to produce negative pressure, resulting in sand mold, external pressure mold sand fastening molding, after placing sand heart, mold, casting, waiting for casting solidification, lift negative pressure or stop pumping gas, The mold sand will then get out of the cast. According to the current application of the field can be divided into vacuum sealing Molding (V method) and real-type vacuum casting (FV method).
Vacuum casting can be roughly divided into vacuum casting, vacuum low pressure casting, vacuum differential pressure casting three kinds:
1, Vacuum suction casting
The mold is placed in a closed container and the air in the mould is drawn out, which causes a certain negative pressure in the mould, which leads to the suction of the metal liquid. When the inner runner of the casting is solidified, the negative pressure is removed, and the molten metal flow in the vertical runner is returned to the pool. Its advantage is to improve the filling capacity of the alloy liquid, the minimum wall thickness of the cast-iron castings can reach O. 2mm, at the same time, casting area 300mm2, while reducing porosity, slag and other defects. Suitable for the production of thin and fine small precision castings, cast steel (including stainless steel) pieces Yu Yi. Its advantages are:
(1) Vacuum casting is advantageous to the elimination of the gas in the mould, to restrain the turbulence and the generation of the rolling gas, to overcome the disadvantage of low pressure casting and differential pressure casting, so that the filling capacity of metal liquid is obviously improved.
(2) The casting temperature of molten metal can be lower than that of gravity casting due to the increase of filling capacity during vacuum suction casting (20~30℃).
(3) Casting wall thickness difference, thin-walled, high quality requirements of castings.
(4) The smooth filling effect can be obtained by selecting the appropriate rate of change of vacuum degree to control the velocity of metal liquid entering the cavity.
2. Vacuum Low Pressure casting
Vacuum low Pressure casting method is in the process of filling the mold vacuum, filling after the pressure to ensure that the casting under constant pressure crystallization solidification, castings are fully reduced, so the casting structure is compact, the mechanical properties are improved. This method is used for precision casting of Al, Mg alloy castings. Its advantages are as follows:
(1) The existence of vacuum negative pressure can make the moulding sand compacted and increase the strength of the mould.
(2) Moderate increase in vacuum, so that the filling time is shorter, not only the filling speed is accelerated and stable.
(3) Vacuum low pressure casting can accelerate the fluidity of aluminum alloy at low temperature, avoid the suction phenomenon of aluminum alloy caused by high temperature casting, and increase the pinhole defect of castings.
(4) Casting formability is good, it is advantageous to form clear contour, smooth surface casting and large-scale thin-walled castings.
(5) The casting is compact and has high mechanical properties.
(6) In general, no riser is required so that the yield of the metal liquid is greatly increased, usually up to 90%.
(7) Good working environment, simple equipment, help mechanization and automation.
3. Vacuum differential Pressure Casting
Vacuum differential pressure casting, mainly is the resistance insulation furnace and mold all sealed, in the sealed cover into a larger pressure such as 500kPa compressed air, at this time because the mold and the crucible inside the pressure is equal, the metal liquid will not rise, and then in the metal liquid surface to add 50kPa pressure, metal liquid will rise filling mold hole. Especially in the production of complex thin-walled castings, but the equipment is large, operation trouble, only when the application of special requirements. Its advantages are as follows:
(1) compared with the traditional gravity casting and vacuum suction casting, the vacuum differential pressure casting has shown a good filling capacity in the 1mm thick sheet specimen.
(2) The castings have a more compact crystallization structure and mechanical properties, and in the casting strength performance, the ratio of vacuum casting and gravity casting is increased by about 50% $number.
(3) Vacuum die-casting can effectively reduce the blowhole of die castings and increase the density of castings by more than 1%.